One thing I have noticed is that a great riff possesses its own “star” quality. In some ways, it is very hard to put your finger on what makes a riff great. You just know that you like it and that it is good.
A perfect example of this is the opening to “Crazy Train”, you agree? The riff bounces around familiar note choices to heavy metal, yet, whenever this riff starts it just pops. This is kind of like a great song. The song might use the same old 3 chords, yet it sounds fresh and inviting to listen to.
So, few thoughts on awesome riffs and what makes them great.
1) The riff has a star quality to it. You know, for whatever reason, that it is awesome.
2) Once the riff ends – you want to hear it start again. The ending of the riff strongly pulls to it being played again. And, you want to hear this as a listener.
3) The performance of a riff is spot on. The execution and attack on the riff is special.
Have you ever noticed when some players play a famous riff and when they play it it sounds like nothing special? That is the performance of it that I am referring to.
4) The sound of the riff is awesome. This is mostly what I refer to the guitar tone. What makes many riffs pop is a combination of guitarist’s style augumented by a great, or at least memorable tone. Most (perhaps all) guitarists have their own tone. That tone changes (improves) as players develop better technique and understanding of the guitar. When you combine personal style and tone in a strong package, you have a very solid ground for potential memorable riffs of your own.